It is known that technical-grade trichlorosilane (SiHCl3, TCS) and/or silicon tetrachloride (SiCl4, STC) is used as a raw material in the preparation of polycrystalline silicon, which finds use, for example, in photovoltaics or in glass fiber or chip production. These products comprise various impurities, for example other silanes, for instance dichlorosilane (SiH2Cl2, DCS), but also, for example, boron compounds, especially boron-trichloride (BCl3).
Boron-containing components are generally detrimental for the further use of TCS and STC, for example in the abovementioned fields of application, and therefore have to be removed or reduced to a target value. Residual amounts of BCl3 in the chlorosilane as a feedstock are an obstacle, for example, to controlled doping of ultrapure silicon. Typical orders of magnitude of impurities are 0.1 to 5 ppm by weight, occasionally also up to 10 ppm by weight or higher.
The removal of trace components by a distillative route is known to be difficult in many applications. For example, the boiling points of boron trichloride (12.5° C.) and, for example, dichlorosilane (8.3° C.) are so close to one another that, in a conventional distillative workup route as shown in FIG. 1, the boron-containing impurities are removed again as so-called low boilers together with the dichlorosilane and a high proportion of trichlorosilane via the top stream of the second distillation column which is used for separation of the top stream of a first distillation column. The low boiler fraction removed in this way, which comprises the boron impurities, especially boron trichloride, is then either discarded in its entirety or is subjected to a nondistillative workup. In the nondistillative workup, for example, complexing methods as described in WO 06/054325 are used. This conventional workup route gives rise to losses of the materials of value, TCS and DCS.
It is therefore an object of the present invention to work up boron-containing chlorosilane streams by a purely distillative process, in such a way that it is possible to obtain high-purity chlorosilane fractions, it being possible to obtain especially either boron-depleted dichlorosilane, trichlorosilane and/or silicon tetrachloride, or else boron-depleted mixtures of dichlorosilane and trichlorosilane, and the loss especially of dichlorosilane and trichlorosilane through removal of boron-enriched silane fractions being reduced.